The present invention relates to an inscribed planetary gear device which is suitable for use as a speed reducer or an overdrive, particularly a speed reducer or an overdrive which is required to have a compact construction and to transmit large power.
Hitherto, an inscribed planetary gear device has been known which has a primary shaft, an externally-toothed gear carried by the primary shaft at an eccentricity, an internally-toothed gear inscribed by the externally-toothed gear, and a secondary shaft connected to the externally-toothed gear through a means which transmits only the rotation of the externally-toothed gear which takes place about the axis of the externally-toothed gear.
An example of this known structure is shown in FIG. 3. This example is intended for use as a speed reducer, with the primary and secondary shafts serving as an input shaft and an output shaft, respectively, and the internally-toothed gear being held stationary.
In the prior art device, an eccentric member 3 is mounted on the input shaft 1, and an externally-toothed gear 5 is fitted on the eccentric member 3. The externally-toothed gear 5 has a plurality of inner roller holes 6 which receive inner rollers 8 carried by inner pins 7.
The externally-toothed gear 5 is provided, on the outer peripheral surface thereof, with teeth of trochoidal or arcuate form. These teeth mesh with internal teeth of an internally-toothed gear 10 which is fixed to a casing 12.
The inner pins 7, extending through the externally-toothed gear 5 are fixed to a flange 14 of the output shaft 2 or tightly fitted in holes formed in the flange 14. One full rotation of the input shaft 1 causes one full rotation of the eccentric member 3. As a result of rotation of the eccentric member 3, the externally-toothed gear 5 tends to revolve around the input shaft 1. The internally-toothed gear 10, however, prevents the externally-toothed gear 5 from rotating about its own axis. As a consequence, the externally-toothed gear 5 revolves while inscribing the internally-toothed gear 10, without making substantial rotation.
It is assumed here that the number of the teeth on the externally-toothed gear 5 is N, while the number of the teeth of the internally-toothed gear 10 is N+1, i.e., that the difference in the number of teeth between these two types of gears is 1. In this case, the rotational phase of the externally-toothed gear 5 is deviated by one pitch of the teeth of this gear 5 with respect to the internally-toothed gear 10 for each full rotation of the input shaft 1. Namely, the externally-toothed gear 5 rotates through an angle corresponding to one pitch of the teeth thereof. This means that one full rotation of the input shaft is reduced to -1/N rotation of the externally-toothed gear.
Oscillating components of movement of the externally-toothed gear are absorbed by the presence of gaps between the inner roller holes 6 and the inner rollers 8 and the inner rollers 8 and the inner pins 7 so that only the rotation of the externally-toothed gear 5 is transmitted to the output shaft 2 through the inner pins 7.
Consequently, a speed reduction at a reduction ratio -1/N can be achieved.
In this known arrangement, the internally-toothed gear is fixed and the primary and secondary shafts are respectively used as the input shaft and the output shaft. This, however, is only illustrative and a reduction can also be achieved by using the internally-toothed gear as the output shaft while fixing the secondary shaft. It is also possible to use the illustrated arrangement as an overdrive by replacing the input and output shafts with each other in the described arrangement.
Conventionally, speed reducers and overdrives employing the described inscribed planetary gear structure are produced substantially by machining alone. Machining has been chosen as the manufacturing method of choice in order to meet the requirements for high dimensional precision of the components, such as the primary and secondary shafts, internally-toothed gear and the externally-toothed gear, as well as for smoothness of the surfaces of these components.
Mass-production by mechanical processing of these components, particularly those for compact speed reducers or overdrives, is difficult to conduct. Consequently, much labor and cost are required in the production of such compact speed reducers and overdrives.
It is possible to fabricate these components by press or die-casting method, but such methods cannot provide required levels of precision and strength, thus failing to meet the requirement for ability of the gears to transmit large amounts of power, although they are suitable for mass-production.
These components also could be formed from plastics by molding. Such a method provides advantages in the production of compact speed reducers or over-drives, such as improvement in the assembly efficiency, decrease in the number of parts and reduction in the production cost. The components molded from plastics, however, cannot provide the amount of strength which is required for transmission of large amounts of power. Furthermore, these components are not usable at high temperatures, which makes it difficult to continuously operate the speed reducer or the overdrive.
It is also possible to fabricate these major components by sintering. This method offers advantages such as reduction in the cost and ease of mass production of small-sized speed reduces or overdrives. Sintered parts, however, generally have low levels of density and, hence, are more fragile than ordinary steel materials. In order to eliminate this problem, it is necessary that the low density sintered parts are post subjected to a heat treatment which tends to degrade the dimensional precision of the sintered parts, often requiring a subsequent mechanical processing, such as machining. Sintering is a kind of press work and, therefore products produced by sintering, inherently have an inferior degree of concentricity. The parts fabricated by sintering, therefore, must be completed to specification by mechanical finishing work in order to attain high degree of axial precision of the parts which is an essential requirement in speed reducers or overdrives of the inscribed planetary gear type to which the present invention pertains.
Furthermore, sintered parts generally have inferior smoothness of surfaces due to use of a material having comparatively large grains. Therefore, a sintered part, e.g., a gear, is likely to encounter problems concerning rolling fatigue strength. The use of a powder of comparatively large grain size, as the material from which a part of a compact speed reducer or overdrive is formed inevitably reduces the modules of the gear teeth, with the result that the formation of the gear teeth is restricted. Furthermore, the pores existing in such a sintered part function as notches which undesirably reduce bending strength at the feet of projections such as teeth.
Sintering also has various other disadvantages. For instance, it is to be noted that only two-dimensional forming is possible by sintering. In addition, it is difficult to obtain parts having stepped configurations by sintering, due to restriction by the use of press jigs. Furthermore, it is not easy to sinter a thin-walled part, e.g., 1.5 mm or thinner, and elongated parts.